Safety of an agricultural vehicle operator is a key issue. In particular, it is important to avoid that rotating shafts, that may be accessible by the operator in case of blockage by the crop or the like, may injure arms, legs or hands when the operator approaches the implement or the trailed machine to remove the cause of the blockage. A safety procedure prescribes that the driving shaft or the PTO shall be disconnected from the traction engine or the power hydraulic power source of the vehicle before the operator is able to approach the blocked implement or trailed machine.
It is known to provide a torque limiter to automatically disconnect torque transmission from the driving shaft of an implement or of a trailed machine when the implement or accessory is blocked e.g. by crop and the operator is driving the vehicle. The torque limiter sets a maximum torque level to be transmitted to the implement or trailed accessory and this avoids that blockage may damage the vehicle or the trailed accessory or both.
Sometimes the operator approaches a blocked implement or trailed machine to remove the cause of blockage, e.g. an obstruction, but the driving shaft of the implement or the PTO attached to the trailed accessory is still running.
Known torque limiters are such that re-engagement is dependent on torque level, i.e. re-engagement occurs when the torque drops below the pre-defined maximum level. This is normally the case when the operator removes the obstruction and the PTO or the driving shaft are not switched off. Sudden re-engagement of the torque limiter may cause severe injuries to the operator if the latter removes the obstruction using his/her feet, hands, arms etc. because the implement or the trailed accessory will immediately start to work again as soon as the torque limiter is re-engaged.
Even in case the PTO is disconnected or the driving shaft is not driven by the traction motor during removal of crop plug, power transmission via torque limiter may be inadvertently started too early. This may be a risk in case the operator is very close to the implement.
In FR 2 316 476, an automatically declutchable drive coupling is described having driving and driven parts. The driven part having inserts whose ends cooperate with ramp surfaces in the driving part. Rollers act as a bearing between the driving and driven parts. A helical spring acts on the inserts through a compression ring. A clutching mechanism for the coupling has inclined zones cooperating with corresponding parts on the inserts, whereby the clutching movement being against the action of a second spring.
In US 2003/136625 a torque limiting coupling is shown having a coupling hub with circumferentially distributed apertures. A switching disk is rotatably arranged between a coupling position and an uncoupling position. The switching disk is loaded towards the coupling position and loads driving members towards the coupling hub. When a torque limit is exceeded, the driving members are transferred into the uncoupling position. A locking means is adjustable between a locking position, where the switching disk is locked in its uncoupling position, and a disengaging position. A retainer is adjustable between a retaining position, where the locking means is secured in its disengaging position, and a releasing position. A detent pawl is movable between a neutral position and a disconnecting position. In the disconnecting position, the detent pawl transfers the switching disk into the uncoupling position. In the disconnecting position, the detent pawl transfers the retainer into the retaining position.
DE 44 41 218 discloses a torque limiting clutch comprising a casing, hub and followers. The clutch is adjustable between a torque transfer and a declutching position. An engaging ring acts on balls, engaging the followers radially inwards in the torque transfer position. If the balls move outwards in the declutching position, they are pressed radially inwards until the ring can slide over them. The ring sliding secures the balls against radially outwards motion by a bore section, thus facilitating the entry of the followers into the recesses and into the torque transfer position with the relative rotation of the clutch hub and casing.